1. Field of the Invention
The invention relates to a semiconductor substrate, comprising a relaxed, single-crystal layer containing silicon and germanium which lies at the surface, as well as a process for producing this semiconductor substrate.
2. Background Art
Prior art has disclosed sSOI and SGOI substrates (strained silicon on insulator and silicon-germanium on insulator, respectively). sSOI substrates and SGOI substrates are distinguished by an electrically insulating layer or an electrically insulating support material. In the case of an sSOI substrate, a thin, single-crystal, strained silicon layer is in direct contact with the insulator. By contrast, an SGOI substrate has one or more layers containing silicon and germanium in a predetermined composition (SixGe1−x with 0<×<1) on the insulator. This layer or combination of layers is also referred to below as a “silicon-germanium layer”. A thin, single-crystal, strained silicon layer can in turn be applied to the surface of the silicon-germanium layer.
In all the known processes for producing sSOI or SGOI substrates, a thin layer consisting of silicon-germanium is separated from a donor wafer by means of mechanical forces, with the free surface of the layer which is to be transferred usually being bonded to a handle wafer before the separation operation. In the case of the sSOI substrate, in addition to the silicon-germanium layer a strained silicon layer is also transferred from the donor wafer to the handle wafer.
The first step of producing an sSOI or SGOI substrate is to prepare a donor wafer. In both cases, a relaxed silicon-germanium layer must first of all be produced on a silicon wafer, and this silicon-germanium layer is in a further step transferred to the handle wafer. Two fundamentally different processes are known for this purpose:
In the first process, a plurality of silicon-germanium layers with an increasing germanium content (graded buffer layer) are deposited epitaxially on the silicon wafer, thereby producing lattice matching between silicon and silicon-germanium. A silicon-germanium layer with a constant germanium content deposited thereon serves for mechanical stress relief, so that silicon-germanium with its natural lattice constant (i.e. relaxed silicon-germanium with a composition of SixGe1−x with 0<×<1) is present at the surface. The surface roughness which is produced during this process can optionally be reduced by subsequent and/or intervening polishing steps. This process requires the epitaxial deposition of layers with a total thickness of approximately 5 μm and is very expensive on account of the long process time associated therewith. Moreover, the process requires a repeated change between epitaxial deposition and polishing, and therefore a large number of individual process steps. The process leads to dislocation densities in the region of 105/cm2.
In the second known process, the layer sequence with a gradually increasing germanium content is dispensed with, and instead a thin silicon-germanium layer of the desired composition is deposited immediately. In this case, the layer thickness is kept below the limit beyond which misfit dislocations are formed. This initially still strained silicon-germanium layer is then relaxed by the silicon-crystal bond which is present directly below the silicon-germanium layer being weakened. This is achieved by implantation of gas ions (for example hydrogen or helium ions) and a subsequent heat treatment. During heat treatment, the implanted ions form gas bubbles which break open the silicon-crystal bond and thus mechanically decouple the silicon-germanium layer and a silicon layer beneath it, which is only very thin, from the remainder of the silicon wafer, which ultimately leads to the relaxation of the silicon-germanium layer. One drawback is the complex implantation step and the formation of microcracks during the formation of gas bubbles, which leads to the destruction of the layer. This process also produces a high dislocation density. Only if an sSOI substrate is to be produced a thin, strained silicon layer is additionally deposited epitaxially on the relaxed silicon-germanium layer.
In the second step of this second process, a superficial layer of the donor wafer (a silicon-germanium layer in the case of SGOI and additionally a strained silicon layer in the case of sSOI) is transferred to a handle wafer. The handle wafer either consists entirely of an electrically insulating material or bears an electrically insulating layer at least at its surface. A number of processes are also known for this second step. The most customary is the process known under the name Smart Cute (EP533551A1). In this process, first of all hydrogen ions are implanted into the surface of the donor wafer. After bonding to a handle wafer, a layer with hydrogen-filled cavities is produced by a heat treatment at approximately 500° C. The separation of this layer is effected by an increasing gas pressure.
In all the known processes for producing sSOI or SGOI substrates, the surface roughness produced during separation of the donor wafer along the prepared separating layer is so high that the substrate cannot be used to fabricate electronic components without any further treatment, for example a polishing or a smoothing heat treatment.